Smart material (also known as intelligent material system) is a type of new executable functional material that can perceive external stimulus, and take some response and action based on the judgment. Smart material is the fourth generation of material following the natural material, synthetic polymeric material, and artificial design material, nowadays new smart material is one of the most important directions of the development of new high-tech materials. Among others, the shape memory material is the most widely studied smart material, as it has some unique properties such as shape memory effect, pseudo-elasticity, and high damping, meanwhile it can recover its original shape or technical parameters by performing action and having some predetermined response to thermal, mechanical, magnetic, or electrical stimulation. These characteristics of the shape memory material have great scientific significance and important engineering significance, so this kind of material has received tremendous attention in the research of new material.
Nowadays the most intensively investigated shape memory material mainly includes shape memory alloy, shape memory ceramics and shape memory polymer. These materials have the advantages of large deformation, having abundant raw materials, great variety of types, wide range of shape memory recovery temperature, light weight, and easy to synthesize, process, use, transport and package. It has been found that many shape memory alloys such as InTi, TiNi, CuAl, CuAlNi, FeNi, FeMn, FeMnC, Fe3Pt, FeMnSi have the effect of two-way shape memory; however, this effect may be achieved by training these alloys in a special way, and the training result may be affected by such factors as the number of training, training temperature, training period, imposed strain force, whether or not treated with thermal cycling and the like, accordingly, the application of two-way shape memory alloy is greatly restricted. Compared with shape memory alloy, the shape memory polymer has the property of shape memory effect and super elasticity effect, so the two way shape memory polymer is often used to achieve the two way shape memory effect. Currently there are mainly three types of two-way shape memory polymer: 1. liquid crystalline elastomers (LCEs); 2. shape memory polymer under constant tensile; 3. shape memory material-elastomer bonded structure. These three types of polymer have their own advantages and disadvantages respectively, for example, the liquid crystalline elastomers type has a high transition temperature and a high cost, and the synthesis procedure is relatively complicated; the second type may lose the effect of two-way shape memory without the external force; the third type can only realize reversible flexural deformation but cannot realize reversible shrinkage. To our best knowledge, so far none of above mentioned methods could realize bidirectional elongation and reversible shrinkage without external tensile at ambient temperature.